1. Field of the Invention:
This invention relates to anhydrous FeF.sub.3 of 99.0% purity or greater (based upon F.sup.- analysis), and a process for its manufacture by the reaction of FeCl.sub.3 and HF under specified conditions. FeF.sub.3 is known as a catalyst in organic reactions and as a burning rate modifier.
2. Statement of Related Art:
Ferric fluoride (FeF.sub.3) is a known substance which was synthesized as early as 1892 by heating HF to 35.degree. C., diluting it with N.sub.2, and then reacting the gaseous HF/N.sub.2 mixture with FeCl.sub.3 [Poulenc, et al., "Seances Acad. Sci." 115:944 (1892)] and "Ann. Chim. Phys. (7)" 2:57 (1894)].
The Merck Index, 9th ed., Merck & Co., Rahway, N.J., U.S.A. (1976) at 3937 states that FeF.sub.3 has a mol weight of 112.85, comprising 50.1% F and 49.49% Fe. The compound forms green hexagonal crystals which, when powdered, gives a distinctive lime green appearance. A common contaminant of FeF.sub.3 is Fe.sub.2 O.sub.3 (ferric oxide) which is a well known reddish pigment, also called "jewelers rouge". [Merck entry 3946].
Kwasnik in "The Handbook of Preparative Inorganic Chemistry", 2nd ed. vol. 1, Academic Press, N.Y., U.S.A. (1963) discloses the preparation of FeF.sub.3 from FeCl.sub.3 and HF.
While there are many other processes for the manufacture or synthesis of FeF.sub.3, many of them yield the hydrate (.1-3 H.sub.2 O) form, which then must be converted to anhydrous form for certain uses. This necessitates further reaction and, in the case of the trihydrate, is very difficult to accomplish.
When anhydrous FeF.sub.3 is produced, it is extremely difficult to obtain in high purity (i.e., free of both contaminant cations and anions, especially of ferric oxides). Known methods for producing anhydrous FeF.sub.3 include:
(A) High temperature and high pressure reactions of F.sub.2 or SF.sub.4 with iron oxyfluoride or iron sulfide [Claverie, et al., "J. Fluorine Chem." 4:52-63 (1974); U.S. Pat. No. 2,904,398; U.S. Pat. No. 2,952,514; and Oppergard, et al., J. Am. Chem. Soc. 82:3835 (1960)].
(B) The reaction of metallic iron with HF and a halogen in the presence of a nitrile [U.S. Pat. No. 4,034,070].
(C) The reaction of FeCl.sub.3 with anhydrous HF or F.sub.2, but without the disclosure of reaction conditions such as temperature, pressure, stoichiometry, etc. [Meshri, et al., Kirk-Othmer Encycl. Chem. Tech., 3d ed. 10:754-5.
(D) The reaction of FeCl.sub.3 with liquid HF under free atmospheric conditions by the slow addition of the liquid HF to the solid FeCl.sub.3 accompanied by removal of (gaseous) HCl by-product, followed by removal of excess HF and acquired H.sub.2 O by heating [see Example 10-Comparison].
However, none of the above processes can consistently produce commercial quantities of anhydrous FeF.sub.3 of sufficiently high purity, the highest being (D) above, which yields 97.0% pure FeF.sub.3 (based upon F.sup.- analysis) but at only about 60 weight percent of theoretical.